1. Field of the Invention
This invention relates to a sheet conveying apparatus for conveying a sheet to the image forming portion of an image forming apparatus, and particularly to a construction for correcting the skew of a sheet such as recording paper or an original.
2. Description of the Related Art
Heretofore, image forming apparatus and image reading apparatuses such as a copying machine, a printer, a facsimile apparatus and a scanner have been provided with sheet conveying apparatuses for conveying a sheet such as recording paper or an original to an image forming portion or an image reading portion. Some of such sheet conveying apparatus are provided with correcting means for effecting the skew conveying correction of the sheet or the positional deviation correction of the sheet to adjust the posture and position of the sheet before the sheet is conveyed to the image forming portion or the image reading portion.
A correcting method by such correcting means uses a pair of registration rollers, and for example, in the case of an image forming apparatus, the so-called loop registration method of ramming the leading edge of a sheet against the nip between the pair of registration rollers being at a halt to thereby form a flexure in the sheet, causing the leading edge of the sheet to be along the nip by the elasticity of the sheet to thereby effect the correction of skew conveying, thereafter rotating the pair of registration rollers at predetermined timing, and synchronize the sheet with an image has become a mainstream.
In such a loop registration method, however, a loop space for forming a loop becomes always necessary, and this has made the apparatus bulky. Also, when the loop space cannot be sufficiently secured, there is the problem that jam due to buckling occurs particularly to a sheet such as thin paper of weak rigidity, or a sound (so-called loop sound) is produced when the sheet is caused to abut against the pair of registration rollers.
Further, there is the problem that the skew conveying correcting capability is changed by the degree of rigidity of the sheet. Specifically, in the case of thin paper low in rigidity, the abutting pressure when the leading edge of the sheet abuts against the registration roller nip is deficient and the leading edge of the sheet may sometimes not sufficiently abut against the pair of registration rollers, and in such case, skew conveying correction cannot be completely effected.
Also, in the case of thick paper or the like high in rigidity, there is the inconvenience that by the shock with which the sheet abuts against the nip between the pair of registration rollers, the sheet goes through the nip between the pair of registration rollers, and if in order to prevent this, an attempt is made to give a load or the like to the pair of registration rollers, for example, by a brake member, it will result in an increase in the cost of the product.
Furthermore, when the leading edge of the sheet is curled or broken, the leading edge of the sheet cannot be accurately along the nip portion of the pair of registration rollers, and this also results in the problem that skew conveying correction cannot be accurately effected and printing accuracy lowers.
On the hand in recent years, the image forming apparatuses and the image reading apparatuses are adapted, by digitizing, to once be capable of reading an original, and thereafter electrically encoding the image information thereof and storing it in a memory. During image forming, they have been designed to read out the information in the memory, and form an image corresponding to the image formation of the original on a photosensitive member by a laser beam or an exposing apparatus such as an LED array and therefore, even in the copying of a plurality of sheets, the mechanical movement of an optical apparatus or the like becomes unnecessary.
Thereby, the inter-sheet spacing which is the interval between a sheet and a sheet can be shortened, and it has become possible to treat many sheets within a short time. As a result, for example, in the case of the image forming apparatus, it has become possible to achieve a substantial improvement in the image forming speed without increasing the process speed during image forming.
However, when as the sheet conveying apparatus, use is made of one adopting the already described loop registration method, design is made such that a sheet is once stopped to form a loop and therefore, the inter-sheet spacing is necessarily determined, and this comes to greatly affect an improvement in the image forming speed (productivity).
So, in order to overcome such an inconvenience, Japanese Patent Application Laid-Open No. Hei 10-067448 proposes a sheet conveying apparatus adopting a registration method adapted to be capable of automatically correct the skew conveying of a sheet.
This sheet conveying apparatus is provided with a pair of conveying rollers (registration rollers) for nipping and conveying a sheet therebetween, a sensor for detecting the skew amount of the sheet provided downstream of the conveying rollers with respect to the conveying direction thereof, and conveying roller inclination correcting means for displacing the conveying rollers so as to be inclined in a direction intersecting with a sheet conveying direction, and when the skew conveying of the sheet is to be corrected, it is adapted to displace the conveying rollers in accordance with the skew of the sheet on the basis of the information from the skew amount detecting sensor to thereby correct the skew conveying of the sheet.
However, in such a conventional-sheet conveying apparatus for displacing the conveying rollers to thereby correct the skew conveying of the sheet, the conveying rollers (registration rollers) are in a posture inclined with respect to the original sheet conveying direction at a point of time whereat the skew conveying correction of the sheet has been effected and therefore, if in that state, the sheet is conveyed, the sheet will be fed in a skew direction with respect to the original sheet conveying direction.
When so-called skew feeding in which the sheet S is thus fed in the skew direction occurs, there is the inconvenience that if this sheet conveying apparatus is applied, for example, to an image forming apparatus, an image will be obliquely transferred to the sheet and printing accuracy will become remarkably inferior.
Against such inconvenience, design is made such that
(1) the pressure contact (nip) between the pair of conveying rollers is released at a point of time whereat the sheet has reached a post-step and the sheet has been held, or
(2) the rotational speed of the pair of conveying rollers is increased at the point of time whereat the sheet has reached the post-step and the sheet has been held, and the loop (flexure) of the sheet is formed between the post-step and the pair of conveying rollers to thereby absorb the skew feeding by the loop.
In the case of item (1) above, however, there is the problem that a mechanism for releasing the nip between the pair of conveying rollers becomes necessary and the cost of the product is greatly increased, or the vibration when the nip is released adversely affects the post-step. Also, there is left the problem that if the sheet is held at the post-step, the nip must be released in a moment and it is very difficult to take the timing therefor.
Also, in the case of item (2) above, there is the problem that when the distance from the conveying roller portion to the post-step portion is short, the absorption of the skew feeding by the loop is very difficult and the sheet becomes wrinkled. Also, when the rigidity of the sheet is high like thick paper or the like, the loop is not formed and the absorption of the skew feeding by the loop is impossible.